1. Field of the Invention
The present invention relates generally to non-invasive and implantable (i.e., invasive) plethysmography methods and devices. The present invention more particularly relates to methods and devices for monitoring volume changes in a limb or tissue segment of a patient. The present invention also relates to methods and devices that calculate blood oxygenation levels.
2. Background Art
Plethysmography is a generic term referring to a variety of techniques for monitoring volume changes, for example, volume changes of the lungs due to respiration, or of blood vessels of a limb or tissue segment. When applied to measurements of blood volume, changes occur in a pulsatile manner with each beat of the heart as blood flows in and out of a portion of the body. The study of vascular activity by fluid displacement methods dates hack to at least 1890. More contemporary techniques include strain gauge, pneumatic, impedance, doppler, and photoelectric plethysmography. A plethysmography device produces a waveform that is similar to an arterial pressure waveform. The waveform is useful in measuring pulse velocity and indicating arterial obstructions.
FIG. 1 illustrates an exemplary plethysmograph 100, which includes a waveform 102 produced by a plethysmography device. For timing reference, an electrocardiogram (ECG) signal 104 is illustrated. Waveform 102 provides a measure of the volume of the arterial vasculature. A measure of arterial pulse amplitude is derived from it. A few tens to a few hundreds of milliseconds after the QRS complex, the plethysmography voltage reaches a minimum and starts to increase. This is due to the increasing blood volume in the arterioles as the systolic pulse reaches the periphery. The delay is influenced by the distance that the sensor is placed from the heart. It requires approximately 100 msec for the waveform to reach its maximum. The excursion from minimum to maximum represents the arterial pulse amplitude. During diastole, the recoil of the elastic arterial vessels continues to force blood through the capillaries, so that blood flows through the capillary bed throughout the entire cardiac cycle.
A photoplethysmography device (PPG) (also called a pseudoplethysmography or photoelectric plethysmography device) includes a light detector and a light source. The PPG utilizes the transmission or reflection of light to demonstrate the changes in blood perfusion. Such devices might be used in the cardiology department or intensive care department of a hospital or in a clinic for diagnostic purposes related to vascular surgery. A photoplethysmography device is also referred to, herein, simply as a plethysmography device.
An exemplary circuit 200A for a conventional photoplethysmography device is shown in FIG. 2A. An exemplary mechanical arrangement 200B for a conventional photoplethysmography device is shown in FIG. 2B. In these examples, the light source is a light-emitting diode (LED) 202, although in alternative models an incandescent lamp can be used as the light source. The light detector in this example is a photoresistor 204 excited by a constant current source. Changes in light intensity cause proportional changes in the resistance of the photoresistor. Since the current through the photoresistor is constant in this example, the resistance changes produce varying analog voltage (Voutxe2x80x94analog) at the output terminal. In order to be useful, this varying analog voltage (Voutxe2x80x94analog) typically must be converted to a digital signal (Voutxe2x80x94digital) using an analog to digital converter (A/D) 206. Other known light detectors include photo diodes, photo transistors, photo darlingtons and avalanche photo diodes. Light detectors are often also referred to as photo detectors or photo cells.
Light may be transmitted through a capillary bed such as in an ear lobe or finger tip. As arterial pulsations fill the capillary bed the changes in volume of the blood vessels modify the absorption, reflection and scattering of the light. Stated another way, an arterial pulse in, for example, a finger tip, or ear lobe, causes blood volume to change, thereby changing the optical density of the tissue. Therefore, the arterial pulse modulates the intensity of the light passing through the tissue. Light from LED 202 is reflected into photoresistor 204 by scattering and/or by direct reflection from an underlying bone structure. Such a PPG does not indicate xe2x80x9ccalibratablexe2x80x9d value changes. Thus, its usefulness is generally limited to pulse-velocity measurements, determination of heart rate, and an indication of the existence of a pulse (e.g., in a finger). Additionally, a conventional PPG provides a poor measure of changes in volume and is very sensitive to motion artifacts.
It is noted that photoplethysmography devices may operate in either a transmission configuration or a reflection configuration. In the transmission configuration, the light source (e.g., LED 202) and the photodetector (e.g., 204) face one another and a segment of the body (e.g., a finger or earlobe) is interposed between the source and detector. In the reflection configuration, the light source (e.g., LED 202) and photodetector (e.g., 204) are mounted adjacent to one another, e.g., on the surface of the body, as shown in FIG. 2B. If the photoplethysmography device is incorporated into an implantable cardioverter defibrillator (ICD) or other implantable therapy device or monitor, and thus implanted, then the light source (e.g., LED 202) and light detector (e.g., 204) can be mounted adjacent to one another on the housing (e.g., can) or header of the ICD, as disclosed in U.S. patent application Ser. No. 09/543,214, entitled xe2x80x9cExtravascular Hemodynamic Sensorxe2x80x9d, filed Apr. 5, 2000, which is incorporated herein by reference in its entirety.
In a conventional photoplethysmography device (e.g., 100A), a constant average optical power is delivered by the optical source (e.g., LED 202) and plethysmograph information (e.g., waveform 102 shown in FIG. 1) is determined based on time varying optical power incident on the detector (e.g., photoresistor 204). This approach is not optimal for many reasons, some of which are discussed above and others of which are discussed below.
First, providing a constant average optical power does not allow for power consumption to be minimized. This may not be a concern if the plethysmography device is a non-invasive device, and thus, can receive power from an relatively inexpensive and inexhaustible power supply. However, if the plethysmography device is an implantable device (or part of an implantable device), as is the case in many embodiments of the present invention, the device is likely powered by a battery that is not easily accessible. For example, if the plethysmograph is incorporated into an ICD, pacemaker, or other implantable therapy device or monitor, then the battery of the device could be used to power components (e.g., LEDs, amplifiers) of the plethysmography device. Typically, invasive surgery is required to replace the ICD or pacemaker when its battery nears depletion. Accordingly, there is a need to minimize power consumption of the components of the plethysmography device. This holds true for any implantable plethysmography device, whether or not it is incorporated into an ICD or pacemaker.
Second, the dynamic range (e.g., linear range) of a photodiode, or any other type of photoresistor or photodetector, is limited. The upper limit is usually a function of the saturation point of the detector and/or detector amplifiers. The lower limit is typically a function of environmental and/or circuit noise. When any of these components are operating outside of its dynamic range, the accuracy and integrity of the information being obtained (e.g., plethysmography waveform 102) is adversely affected. Accordingly, there is a need to operate a plethysmography device within the dynamic range of its components. The criticality of this need is increased when the plethysmography device is an implantable device (or part of an implantable device). This is because, over time, the human body begins to encapsulate an implanted device with a fibrous capsule. Such a fibrous capsule can change the amount of light that reaches the detector of a plethysmography device. This can cause components (e.g., a photodiode and/or amplifier) to operate outside of their dynamic range, thereby adversely affecting the accuracy and integrity of the plethysmography information. This further increases the need to operate a plethysmography device within the range of its components. Accordingly, there is a need to compensate for the fibrous capsule that, slowly, over time, encapsulates an implanted plethysmography device.
Third, as mentioned above, the varying analog voltage output (e.g., Voutxe2x80x94analog) of a conventional plethysmography device typically must be converted to a digital signal (e.g., Voutxe2x80x94digital) using an analog to digital converted (e.g., A/D 106). Such an analog to digital convert consumes power to perform its conversion. As also mentioned above, minimizing power consumption is very important when the plethysmography device is implanted. Accordingly, it would be beneficial to avoid the necessity of an analog to digital converter, to thereby reduce power consumption.
The present invention, which relates to monitoring volume changes in blood vessels, is directed towards methods for use with a medical device that includes a light source and a light detector. A time-varying modulating signal is used as a plethysmography signal, rather than a time-varying detected optical power. The time-varying detected optical power is used (e.g., in a feedback loop) to adjust the source intensity.
According to an embodiment of the present invention, light is transmitted from the light source. An intensity of the transmitted light is based on a light control signal. The light detector receives a portion of the light transmitted from the light source. The received portion has an associated detected light intensity. A feedback signal is produced based on the portion of light received at the light detector. The feedback signal is indicative of the detected light intensity. The feedback signal is compared to a reference signal to produce a comparison signal. The light control signal is adjusted based on the comparison signal. At least one of the comparison signal and the light control signal is representative of volume changes in blood vessels.
According to an embodiment of the present invention, the light control signal is adjusted to keep the detected light intensity of the portion of light received at the light detector relatively constant. An amplitude of the light control signal can be adjusted to keep the detected light intensity of the portion of light received at the light detector relatively constant. According to another embodiment of the present invention, a frequency of the light control signal is adjusted to keep the detected light intensity of the portion of light received at the light detector relatively constant. In another embodiment, a pulse width of the light control signal is adjusted to keep the detected light intensity of the portion of light received at the light detector relatively constant.
According to an embodiment of the present invention, the light control signal is adjusted to minimize a difference between the feedback signal and a reference signal. The amplitude, frequency and/or pulse widths of the light control signal can be adjusted to minimize the difference between the feedback signal and the reference signal.
In an embodiment of the present invention, the light control signal includes a digital signal, and the light source includes a plurality of LEDs, each of which is on or off based on the digital signal. The digital signal is adjusted to minimize a difference between the feedback signal and the reference signal, or to keep the detected light intensity of the portion of light received at the light detector relatively constant.
The light source can transmit light of more than one wavelength. For example, in an embodiment of the present invention the light source transmits light having a first wavelength and light having a second wavelength. An intensity of the transmitted light is based on a first light control signal and a second light control signal. The light detector receives a portion of the light having the first wavelength and a portion of the light having the second wavelength transmitted from the light source. Each portion has an associated detected light intensity. A first feedback signal is produced based on the portion of light having the first wavelength. A second feedback signal is produced based on the portion of light having the second wavelength. Each feedback signal is indicative of the associated detected light intensity. The first feedback signal is compared to a reference signal to produce a first comparison signal. The second feedback signal is compared to a reference signal to produce a second comparison signal. The first and second reference signals may or may not be the same signal. The first and second light control signals are adjusted, respectively, based on the first and second comparison signals. The first and second comparison signals are representative of volume changes in blood vessels. In an embodiment of the present invention, blood oxygenation levels are calculated based on the first and second comparison signals. In an embodiment, the first wavelength is within the red visible light spectrum, and the second wavelength is within the infrared or near infrared light spectrum.
The devices of the present invention can be non-invasive or implantable. An embodiment of the present invention is directed to an implantable device for monitoring volume changes in blood vessels. The implantable device includes a light source adapted to transmit light. The implantable device also includes a light detector adapted to receive a portion of the light transmitted from the light source. The portion of light received at the light detector has an associated detected light intensity. A light controller of the implantable device is adapted to adjust an intensity of the transmitted light based on the detected light intensity. The device also determines volume changes in blood vessels based a signal produced by the light controller, wherein the signal is proportional the intensity of the transmitted light. The light source and the light detector can be located adjacent to one another on the can or header of the implantable device. The light controller is preferably located within a sealed portion of the implantable device.